The present invention relates to diesel engines, and more particularly to diesel particulate filter (DPF) regeneration.
Diesel engines have higher efficiency than gasoline engines due to the increased compression ratio of the diesel combustion process and the higher energy density of diesel fuel. As a result, a diesel engine provides improved gas mileage than an equivalently sized gasoline engine.
However, the diesel combustion cycle produces particulates that are preferably filtered from the exhaust gases. A diesel particulate filter (DPF) is usually arranged in the exhaust stream to filter the diesel particulates from the exhaust. In order to maintain a desired function of the DPF it must be regenerated at regular intervals to remove the trapped diesel particulates. During regeneration, the diesel particulates are burned within the DPF to enable the DPF to continue its filtering function.
According to one method, regeneration is carried out by injecting diesel fuel into the cylinder after the main combustion event. The post-combustion injected fuel is expelled from the engine with the exhaust gases and is combusted over catalysts placed in the exhaust stream. The heat released during the fuel combustion on the catalysts increases the exhaust temperature, which burns the trapped soot particles in the DPF. This approach utilizes the common rail fuel injection system and does not require additional fuel injection hardware. An arrangement for carrying out this type of regeneration is known from U.S. Pat. No. 7,104,048.
FR 2 900 964 discloses an internal combustion engine provided with an exhaust gas after treatment system. In said document compressed air from the turbo compressor is injected into the exhaust system for increasing the wetting area of the fuel which is provided upstream the catalytic system.
Engine emissions are typically reduced by lowering cylinder temperatures, which occur when ignition timing is retarded. Retarding ignition timing, however, triggers the combustion process at a non-optimal point. As a result, engine efficiency, fuel economy and/or performance are reduced.
Exhaust gas recirculation (EGR) is another, more preferable method for reducing engine emissions. EGR involves re-circulating exhaust gases back into the cylinders, which limits the amount of oxygen available for combustion and lowers cylinder temperatures. EGR enables ignition timing to remain at an optimum point, which improves fuel economy and/or performance.
A problem with the use of conventional fuel injection into the cylinders is that the EGR system must be disabled during DPF regeneration to prevent the post-injected fuel from being re-circulated into the engine. The re-circulation of the post injection fuel may damage the engine and/or the EGR system. Because the EGR system is disabled during DPF regeneration, the engine emission rates may increase during DPF regeneration.
According an alternative method, DPF regeneration is carried out by injecting diesel fuel directly into an exhaust conduit upstream of the catalysts and the DPF using a separate fuel injector. This arrangement does not interfere with the operation of the EGR system. However, a problem with this method is that the diesel fuel causes wetting of the inner walls of the exhaust conduit. As the diesel fuel is injected a shorter distance upstream of the catalysts, the injected fuel has less time to evaporate. A delayed evaporation of the diesel fuel may cause a reduced heat release from the catalysts and thereby an insufficient DPF regeneration. This may in turn force the system to inject a larger amount of fuel or to carry out additional DPF regenerations to maintain a satisfactory function of the DPF.
It is desirable to provide an improved method for DPF regeneration that will avoid the above problems and ensure a sufficient heat release from the catalysts to achieve sufficient cleaning of the DPF even at low engine load.
The invention relates, according to an aspect thereof, to a method for operating a diesel engine provided with a particulate filter and a diesel engine system with means for regenerating such a particulate filter.
According to a preferred embodiment, the invention relates to a method of operating a diesel engine provided with an exhaust conduit comprising a controllable supercharger located upstream of a catalytic converter, a fuel injector located downstream of the supercharger and a diesel particulate filter located downstream of the catalytic converter. The method may comprise the steps of:                determining that a regeneration cycle is required for the diesel particulate filter;        injecting a predetermined amount of fuel over a period of time into the exhaust conduit upstream of the catalytic converter;        controlling the flow rate of exhaust through the supercharger during the period of fuel injection;        combusting said fuel to burn particulates trapped within the diesel particulate filter.        
Regeneration of the diesel particulate filter, hereinafter referred to as the DPF, is carried out by injecting diesel fuel directly into an exhaust conduit upstream of the catalysts and the DPF using a separate fuel injector. Fuel under pressure may be supplied from an existing fuel rail or a dedicated fuel pump drawing diesel fuel from a tank. This arrangement does not interfere with the operation of an existing EGR system. The injected fuel is combusted in the catalytic converter and the heat released by this combustion will cause the-particulate matter, or soot, in the DPF to combust and burn.
When the diesel fuel is injected into the exhaust conduit a certain amount of wetting of the inner surface of the conduit may occur, whereby a film of said fuel is deposited on the wall of the conduit. Wetting is most likely to occur during engine operating conditions where the temperature of the inner surface is relatively low. Examples of such conditions may be periods of relatively low engine load, when the vehicle is stationary with the engine idling, when the vehicle is operated at a constant, relatively low speed, or when the vehicle is coasting. Depending on the relative location of the components in the system, a low ambient temperature may also affect the rate of evaporation.
When the flow rate is constant, injected fuel will cause wetting of a relatively small area over a limited section of the inner wall of the exhaust conduit. If the flow rate is maintained constant, a relatively thick film of fuel will be deposited on said limited section of the wall. At low temperature conditions this film may take a relatively long time to evaporate. As the heat release to the exhaust as the evaporated fuel is combusted in the catalytic converter will take place over a longer period of time, the temperature of the exhaust downstream of the catalytic converter will be correspondingly lower. This may result in an incomplete combustion of the particles in the DPF. As stated above, it may be necessary to increase the amount of injected fuel or to carry out the regeneration cycle more frequently to overcome these problems.
As the diesel fuel is injected a relatively short distance upstream of the catalytic converter, it is desirable to ensure that the injected fuel has evaporated before it reaches the catalytic converter. It is also desirable that the evaporation occurs as quickly as possible after the fuel injection in order to obtain a sufficient heat release from the catalytic converter to completely combust the particles collected in the DPF. This is achieved by controlling the supercharger to vary the flow rate of exhaust while the fuel is being injected. By varying the flow rate of the exhaust, the area of the inner wall subjected to wetting is enlarged and the film of fuel will evaporate at a satisfactory rate.
According to a first example, the control unit may control the supercharger to increase or decrease the flow rate of the exhaust continuously during the period of fuel injection. By increasing the flow rate from an initial value, the area of wetting will be displaced downstream relative to a first area of wetting where a film of injected fuel will be deposited at the initial flow rate. Similarly, by decreasing the flow rate from an initial value, the area of wetting will be displaced upstream relative to said first area of wetting.
According to a second example, the control unit may control the supercharger to pulse the flow rate of exhaust continuously during the period of fuel injection. When pulsing the flow rate, an initial flow rate will be increased in a first step, whereby the area of wetting displaced downstream relative to the first area of wetting. Subsequently, the flow rate will return to the initial flow rate. In a second step, the flow rate will be decreased whereby the area of wetting displaced upstream relative to the first area of wetting. Subsequently, the flow rate will return to the initial flow rate. Either of the first or second steps may be used as the initial step. One or more such pulses, comprising a first and a second step, may be carried out in sequence.
The rate of control of the flow rate of the exhaust in the above examples is dependent on a number of parameters, such as the minimum and/or maximum allowable flow rate through the supercharger in relation to the initial flow rate, the type of supercharger used, the size and shape of the exhaust conduit downstream of the fuel injector, the relative distance between the first area of wetting and the catalytic converter, etc.
Changing the flow rate through the supercharger may also affect the direction of flow, for instance a rotary component of the flow referred to as swirl. Mixing of the exhaust gas and the fuel injected into the exhaust conduit is enhanced when the flow is turbulent. The swirl component can be influenced by a number of factors, such as the shape of the exhaust conduit, the exhaust flow through the supercharger or the positioning of the fuel injector. When the flow rate of the exhaust is increased, the swirl component in the exhaust stream may also increase. An increased swirl component may enlarge the area of wetting described above, which in turn increase the rate of evaporation.
Suitable superchargers for this purpose may be a variable geometry turbocharger (VGT) or an electrically driven or otherwise controllable turbo charging unit located in the exhaust conduit downstream of the engine.
According to a third example, the control unit may control an engine related parameter or auxiliary components driven by the engine in order to vary the mass flow rate and/or the temperature of the exhaust through the supercharger during the period of fuel injection. This may be achieved in a number of different ways, wherein a few examples are listed below. For instance, the exhaust flow rate may be varied by controlling the fuel injection into each cylinder. Alternatively the flow rate is varied by instructing the control unit to actively change the engine speed, which allows regeneration when the vehicle is stationary. The engine speed, and thereby the exhaust flow rate, can also be varied by activating an auxiliary load, such as a generator, radiator fan or air-conditioning unit. According to a further example, the control unit may control a number of actuators in the intake and/or exhaust conduits, such as a throttle in the air intake or an EGR valve in the exhaust conduit.
One or more of the above examples may be used for controlling the supercharger itself or the flow rate of exhaust through the supercharger to vary at least the flow rate during the period of fuel injection during regeneration of the DPF.
The regeneration process may be controlled with respect to a known or predetermined exhaust flow rate. When the flow rate is known, a calculated or experimentally determined amount of fuel required for regenerating the DPF is injected into the exhaust conduit over a predetermined period of time. The period of time may be dependent on the rate of evaporation for a current temperature of the exhaust conduit, the temperature of the catalytic converter and/or the method selected for controlling the flow rate of the exhaust. Using the above method allows the amount of injected diesel fuel required for regeneration of the DPF to be minimized. At the same time, a substantially complete regeneration of the DPF allows the period of time between each consecutive regeneration to be maximized.
The method is preferably, but not necessarily carried out when it has been determined that the engine is operated in a steady state. The control unit may carry out this step prior to performing the injection, using existing sensors used for monitoring various engine related parameters. The regeneration may be carried out automatically under the control of the control unit, when a regeneration of the DPF is required. Alternatively, manual initiation of the injection by the driver may be allowed when the control unit has determined that a regeneration of the DPF is required and that the engine is operated in a steady state. The state of the DPF is monitored by the control unit and regeneration is preferably carried out immediately prior to the time when it is determined that the DPF is full.
The invention also relates to a diesel engine system for carrying out the above method. The diesel engine system comprises a diesel engine provided with an exhaust conduit comprising a controllable supercharger located upstream of a catalytic converter, a fuel injector located between the supercharger and the catalytic converter, a diesel particulate filter located downstream of the catalytic converter, and a control unit for controlling the engine system. A control unit is arranged to determine that a regeneration of the particulate filter is required, which regeneration is initiated when a predetermined condition is fulfilled. The fuel injector is arranged to inject a predetermined amount of fuel during a first stage of the regeneration. At the same time, the supercharger is arranged to vary the flow rate of exhaust during the injection of fuel. Finally, the catalytic converter is arranged to combust the injected fuel, and that the combusted fuel is arranged to burn particulates trapped within the diesel particulate filter.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.